The present invention relates to a method of controlling, with the aid of a field of characteristic curves of engine variables, the modulation pressure in an automatic transmission in a motor vehicle having a hydraulic gear shift system.
The modulation pressure controls the built up hydraulic pressures for the entire operation of an automatic transmission system. Through the modulation pressure hydraulic regulating valves are controlled which change the gear shift of the automatic transmission.
In a known automatic transmission the hydraulic gear shift system is operated in dependency on engine load and road speed of the vehicle. The load and the speed are determined mechanically, namely the load is determined from the gas pedal position and the road speed by a centrifugal governor. From these variables the modulation pressure is adjusted and a suitable gear is set accordingly. This known method of controlling the modulation pressure has the disadvantage that it is inaccurate, requires too many mechanical parts and consequently is susceptible to failures.
Known is also to influence the modulation pressure by electromagnetic control valves which in turn are controlled by an electronic control circuit including one or more fields of characteristics of engine variables. In the stored fields of characteristics the engine speed and load can be used as controlling variables. To achieve a sensitive setting of the gears a large number of reference data has to be stored in the fields of characteristics. It has been devised to use a three-dimensional storage matrix or field of characteristics for controlling the modulation pressure. However, both above discussed kinds of control of the hydraulic or electromagnetic valves are very expensive especially when it is desired to provide a sensitive gear changing operation. A particularly disadvantageous feature of these known controls is the necessity to use a very large storage matrix for reference data assigned to the sets of engine variables.